A few posts back Heraclides referred to a kitten born with two heads - you may well have seen it on TV news the other night. (If you google 'two-headed kitten' you'll find this wasn't a unique birth. I remember we had a two-headed calf in the zoology museum at Massey, when I was a student there.)
Here's an image from one of the Australian news sites:
Apparently the wee thing meows & purrs with both mouths, although it feeds through only one as the other has a cleft palate. And yes, you probably looked at this & went, eeeeugh! But there's an interesting question here - how does this sort of teratology (birth defect) develop?
To me - & I hasten to add that I'm no expert! - this looks rather like one form of conjoined twinning. Armand Leroi's book Mutants kicks off with an examination of this phenomenon in humans. (It has a sort of ghastly fascination, really, which I remember feeling as I watched the associated BBC program.)
An embryo first forms as a solid ball of tiny cells, produced by repeated divisions of the original zygote. But fairly early on, this ball becomes hollow, at which stage it's called a blastula. Then the blastula starts to invaginate - some of its cells start to flow inwards, through an opening called a blastopore, to line an internal cavity that will ultimately become the gut.
Leroi explains that a small group of cells at the lip of the blastopore play a very important role in the future development of the embryo. This tiny bit of tissue, called an 'organiser', produces a chemical gradient that signals to the cells moving past it what and where their future role will be. In other words, it 'organises' the embryo. The signalling molecules involved are given whimsical names such as 'noggin', which plays a role in the development of the head & brain. And interactions between the different signalling molecules determine the ultimate fate of the early embryo's cells. Several sets of experiments - early ones in the 1920s involving microsurgery on newt embryos, and more recently using the signalling proteins themselves - have found that if you have 2 organisers, you'll end up with conjoined twins. So maybe a developmental error, giving the effect of two organisers, is what produced that little two-headed cat?
Such defects also raise an interesting philosophical question (& one which was addressed by the late Stephen Jay Gould in one of his essays): at what point does one individual, become two? One kitten, two heads. Two heads - two brains? We can't actually answer that without a scan. But if two brains, then - two individuals? Two personalities? Gould raised the question in a discussion of a pair of French conjoined twins, Ritta-Christina, who were a little further along the route to 'separate-ness' than this kitten: they had 2 heads, 4 arms, 2 spines - but their spines fused at a single pelvis, below which there were 2 legs... Theirs is a sad story - they were born to poor Sardinian parents, at a time when people were prepared to pay to see 'monsters' & other human curiosities. The family ended up in Paris, where scientists were continually asking to have the children uncovered & undressed so that they could be studied. But there was very little financial support forthcoming, & the family lived in fairly squalid conditions. Eventually the inevitable happened: Ritta, who had always been the weaker of the two, caught a chill that turned to serious illness, and the girls died when just 9 months old. By all accounts the doctors & scientists were on the doorstep almost as soon as this happened, asking first to make a plaster cast of the girls' bodies, but shortly afterwards wanting the body itself* for dissection. It would be nice to think that we live in a more enlightened age...
* difficult, isn't it? There was, after all, just a single corpse. And yet everyone who saw them recognised that Ritta & Christina were two distinct individuals.
A.M. Leroi (2003) Mutants: on the form, varieties and errors of the human body. Harper Perennial.